Dual-layer coating containing aluminum-flake pigment and coated article

ABSTRACT

A high-solids enamel coating on a substrate can give metallic glamor comparable to that imparted by conventional lacquer coatings containing metallic-flakes when the amount of metallic flake pigment in the enamel coating composition is increased over conventional amounts with a proportional increase in the amount of chromatic pigments.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 612,658, filed May 21,1984, now abandoned, which is a continuation of application Ser. No.420,510, filed Sept. 20, 1982, now abandoned, which is acontinuation-in-part of U.S. application Ser. No. 080,084, filed Sept.28, 1979, now U.S. Pat. No. 4,359,504.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention is related to dual-layer high-solids enamel coatingsystems containing aluminum-flake pigment which, when applied to asubstrate and dried, result in coatings having excellent metallicglamor.

2. Description Of The Prior Art

Metallic glamor in the coating art refers to the property of ametallic-pigmented coating that causes the intensity of light reflectedfrom the coated substrate to vary markedly according to the angle fromwhich it is observed. This aesthetic property, which is particularlydesired in automotive finishes, is largely the result of a non-randomorientation of the metallic flakes in the dried coating, with most ofthe flakes positioned practically parallel to the coating surface.Heretofore, such orientation has been normally achieved only withlow-solids lacquer or enamel coating systems which, although ofexcellent quality, have relatively high solvent content. Coatingcompositions in which the solvent content is low, usually 50% by weightof the composition or less, are normally enamel systems. Thesehigh-solids enamel systems, however, have been unable to achievehigh-quality metallic glamor. The current environmental emphasis onsolvent reduction highlights the continued need for a high-solidscoating system which provides metallic glamor comparable to that of thelacquer systems.

SUMMARY OF THE INVENTION

The present invention provides an article having a substrate coated withan improved dual-layer high-solids enamel coating of the kind whereinthe coating comprises

(A) a cured base layer produced from a composition having

(1) binder constituents consisting essentially of a first film-formingmaterial and a crosslinker for the film-forming material wherein thefilm-forming material is selected from the group consisting of acrylicresins, polyester resins, and alkyd resins having a weight-averagemolecular weight of 500-10000;

(2) a solvent for the binder constituents in an amount of no more than100% by weight of the binder; and

(3) chromatic and metallic-flake pigments;

and

(B) a cured transparent top layer, deposited on said base layer,produced from a composition having

(1) binder constituents consisting essentially of a second film-formingmaterial and crosslinker for the film-forming material wherein thefilm-forming material is selected from the group consisting of acrylicresins, alkyd resins, and polyester resins having a weight-averagemolecular weight of 500-10000; and

(2) a solvent for the binder constituents in an amount of no more than100% by weight of the binder,

wherein the improvement is characterized in that the metallic-flakepigment is present in the base layer in an amount of 6-24% by weight,based on the weight of the binder constituents of the base layer, andthe chromatic pigment is present in an amount sufficient with the amountof the metallic-flake pigment to give the dual-layer coating a flakeorientation index of at least 40.

DETAILED DESCRIPTION OF THE INVENTION

The improved dual-layer, high solids, pigmented coating composition ofthis invention achieves metallic glamor of a high quality which hasheretofore not been produced in high-solids enamel systems. Metallicglamor is seen to an observer examining a coated substrate as aprogressive darkening of the apparent color as the observer's viewingangle shifts from the normal. This effect results from an orientation ofa great majority of the metallic flakes in the coating to positionswithin very small angles of the coating surface.

It is well known in the art that this preferred flake orientation islargely produced by shrinkage of the coating in a direction normal tothe substrate that occurs as the volatile components of the coatingcomposition evaporate. In conventional solution lacquer systems, forexample, this shrinkage accounts for a 66% loss in thickness duringsolvent evaporation. In high-solids enamel systems, defined for purposesof this invention as coatings containing no more than 50% by weight ofsolvent, based on binder and solvent, the shrinkage can be as low as28%, and the desired leveling action on the flake is diminished.

Metallic-flake pigments are usually present in coating compositions inan amount of about 0.5-2.5% by weight, based on the weight of thebinder. Automotive topcoats, for example, also have a significant amountof chromatic pigment, the amount often varying according to the desiredcolor. The usual levels of flake pigmentation, however, have been unableto reproduce the metallic glamor in high-solids enamels that has beenachieved with lacquer coatings or low-solids enamels. It has now beenfound, however, that high-solids enamel coating systems of the kindhaving a pigmented base layer overlaid with an unpigmented top layer canachieve the desired metallic glamor when the amounts of bothmetallic-flake pigment and chromatic pigment in the base layer areincreased by a factor of about 3-15 over the amounts of such pigmentsnormally used in a particular metallic color system.

The present invention contemplates that the metallic flake be present inthe pigmented base layer in an amount of about 6-24% by weight, based onthe weight of the binder of the base layer. Preferably 10-20% by weightof flake is present. To impart the desired metallic glamor, thechromatic pigments must be present in an amount sufficient to hide thesubstrate with a color intensity that can appear dark enough, at viewingangles shifted from the normal, to provide the necessary contrast.Usually the chromatic pigment is present in the base layer in thisinvention in an amount of 0.5-85% by weight based on the weight of thebinder of the base layer. The exact amount depends somewhat on thedegree of transparency of the coating at various loadings of thechromatic pigment, which is in turn dependent on the color of thepigment.

Metallic glamor can be objectively measured with a specialgoniophotometer using the following geometrical arrangement. A coatedtest panel is positioned horizontally within the goniophotometer withthe coated side facing up. The light from a circular, concentrated lightsource is collimated by a lens and is directed to strike the panel at asmall angle, typically 22.5°, from the normal. The light beam reflectedfrom the surface of the panel is directed by a second lens, at unitymagnification, through a circular aperture having approximately the samediameter as the light source. A photocell is positioned at a distance ofabout 6-10 aperture diameters behind the aperture and is of sufficientsize to intercept all light reflected from the panel through theaperture. The panel can be rotated to different viewing angles about anaxis that is defined as the intersection of the plane of the panel andthe plane defined by the beam of light incident to and reflected fromthe panel in its original horizontal position. It has been foundparticularly useful to measure the intensity of the reflected light attwo different panel positions, when the panel has been rotated topositions of +10° and +60° from its initial horizontal position.

The goniophotometer gives unitless numerical readings known as luminousreflectance (G) for each angle from which the panel is viewed. Thephotocell is calibrated, with respect to the light source, to indicate aluminous reflectance of 100 for a nonmetallic, matte-surfaced, perfectwhite viewed at any angle setting. Nonmetallic coatings give equivalentreflectance readings from any angle of view.

In a coating having good metallic glamor, the reflectance measured at10° will be greater than that measured at 60°. The reflectance at agiven angle A° is related to another visual characteristic known aslightness (L) through the equation

    L(A°)=25.29G(A°).sup.1/3 -18.83

Goniophotometry, reflectance, and lightness are generally explained inThe Measurement of Appearance, Hunter, R. S., John Wiley and Sons, NewYork 1975.

An objective characterization of the metallic glamor, the flakeorientation index (F.O.I.), is in turn expressed as a function of thelightness of the coating at 10° and 60°. The mathematical expression is##EQU1## The denominator of this expression is a normalizing termempirically determined to produce equal F.O.I. values for all coatingshaving the same metallic glamor, regardless of the color of the coating.It accounts for the color intensity of the coating and the ability ofthe coating to hide the substrate, both dependent on the amount ofchromatic pigment. An index of at least 40, preferably 45, is desirable.

By way of comparison, commercial solution lacquer systems that havevisually acceptable metallic glamor exhibit an F.O.I. of about 50. Whenthe same flake-to-binder and chromatic pigment-to-binder weight ratiosas used in this lacquer system are used in a conventional high-solidsenamel system, the coating exhibits an unacceptably low F.O.I. of 29. Amost preferred embodiment of the present invention, a blue metalliccoating containing 16-20% by weight of aluminum flake and about 5% byweight of blue chromatic pigment (both based on weight of binder ofpigmented base layer), exhibits a flake orientation index of 49. Thiscoating has the same binder composition and binder-solvent ratio as theabove conventional high-solids enamel, but the pigmentation level, flakeand chromatic in the base layer is increased by a factor of 12.

The metallic-flake pigments used in the present invention can be any ofthe conventional flat metallic flakes. Examples are aluminum flakes,nickel flakes, tin flakes, silver flakes, chromium flakes, stainlesssteel flakes, gold flakes, copper flakes, or combinations of these.Preferred are aluminum flakes of the kind described, for example, inU.S. Pat. No. 2,662,027.

The chromatic pigments can be any of the conventional pigments used incoating compositions. Examples are iron oxide; metal hydroxides;sulfides; sulfates, carbonates; carbon black; phthalocyanine blues andgreens; organo reds, and other organic dyes.

The pigments, both chromatic and metallic-flake, can be introduced intothe coating composition by first forming a pigment concentrate or millbase with the film-forming resin, or with polymers compatible with thefilm-forming resin, used in the binder of the base layer. Themetallic-flake pigment concentrate or mill base is preferably preparedby prolonged stirring with the polymeric portion of the concentrate ormill base. The chromatic pigment concentrate or mill base can be formedby conventional techniques such as sand grinding, ball milling, attritorgrinding, or two-roll milling, to disperse the pigment in the vehicle.The pigment concentrate or mill base is then blended with the bindermaterial in amounts appropriate to give the desired pigmentation levels.

The binder constituents of the two layers of the dual-layer enamelcoating can be the same or different. Although the improvements of thepresent invention, imparted by the higher levels of metallic-flakepigments, would be evident in any binder system (film-forming resins andcross-linker), it is preferred that the binder systems have film-formingresins selected from the group of acrylic, alkyd, and polyester resins.

The acrylic resins useful as the film-forming resins in this inventionare hydroxyl-functional copolymers having a weight average molecularweight of about 500-10000 determined by gel permeation chromatography.The copolymers are primarily of alkyl methacrylates and hydroxyalkylacrylates or methacrylates, but may contain other monomerscopolymerizable therewith.

The alkyl methacrylates useful in such resins contain 1-18 carbon atomsin the alkyl group. Typical are methyl methacrylate, ethyl methacrylate,butyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, nonylmethacrylate, lauryl methacrylate, stearyl methacrylate, cyclohexylmethacrylate, isodecyl methacrylate, propyl methacrylate, phenylmethacrylate, and isobornyl methacrylate.

The hydroxy-functional monomers useful in the copolymer arehydroxy-alkyl esters of acrylic or methacrylic acid having 2-10 carbonatoms in the alkyl group. Preferred, however, are esters having 2-4carbon atoms in the alkyl group with the hydroxyl group in the betaposition. Examples are 2-hydroxy ethyl acrylate, 2-hydroxy propylacrylate, 2-hydroxy butyl acrylate, 2-hydroxy ethyl methacrylate,2-hydroxy propyl methacrylate, and 2-hydroxy butyl methacrylate.

The other monomers that can be copolymerized with the alkylmethacrylates and hydroxy-alkyl acrylates and methacrylates to formcopolymers useful in this invention are vinyl acetate; olefins such asethylene, propylene, and the like; conjugated dienes having 4-10 carbonatoms; aromatic hydrocarbons having vinylene groups, such as styrene;alkyl maleates; and alkyl esters of acrylic acid having 2-18 carbonatoms in the alkyl group. Small amounts of ethylenically unsaturatedcarboxylic acids can also be present, such as acrylic acid, methacrylicacid, crotonic acid, maleic acid, itaconic acid, and the like.

Crosslinking agents used with the hydroxyl-functional copolymers can beaminoplast resins or organic polyisocyanates. Preferred are aminoplastssuch as alkoxymethyl melamines.

The aminoplasts are the alkylated products of amino resins, the latterprepared by the condensation of at least one aldehyde with at least oneof urea, N,-N'-ethyleneurea, dicyandiamide, and an aminotriazine such asa melamine or a guanamine. Among the aldehydes that can be used areformaldehyde and its revertable polymers, acetaldehyde, crotonaldehyde,and acrolein.

The preferred aminoplasts are formed by alkylating the amino resins withfrom one to six alkanol molecules, each containing 1-6 carbon atoms. Thealkanols can be straight-chain, branched, or cyclic. The most preferredmembers of this class are the methylated melamine-formaldehyde resinssuch as hexamethoxymethylmelamine.

The organic polyisocyanates that can be used as the crosslinker includealiphatic, cycloaliphatic, alkaryl, aralkyl, heterocyclic, and aryl di-or tri-isocyanates. Oligomers of these can also be used. Preferred fromthese classes are hexamethylene di-isocyanate or its trimer,methylene-bis-(4-cyclohexyl isocyanate), and isophorone di-isocyanate.

The relative amount of cross-linker to film-forming resin will dependlargely on the amount of hydroxyl monomer in the copolymer. Normally,however, high solids enamel compositions have acceptable physical andchemical properties when the stoichiometrically necessary amount ofcross-linker is from 5-50% of the total binder weight.

The alkyd resins which are useful as the film-forming resins of thisinvention are well known as including esterification products havingfatty acid or oil constituents. The resins are usually the reactionproduct of a fatty acid or its glyceride, a polyol, and a polybasicacid. Alternatively, the alcoholosis product of a fatty acid glycerideand a polyol can first be formed and then further reacted with a polyoland polybasic acid by well-known methods to produce resins having aweight average molecular weight, determined by gel permeationchromatography, of about 500-10000. The alkyd resins can be formed fromvarious mixtures of several different fatty acids, polyols, andpolyacids.

The fatty acid glycerides are found in castor oil, dehydrated castoroil, coconut oil, cottonseed oil, peanut oil, tung oil, linseek oil,soya oil, etc. The fatty acids derived from these and like oils arestraight-chain monocarboxylic acids having a total of 8 to 12 carbonatoms. The acids can be saturated or mono- or polyunsaturated. When theacids are unsaturated, derived from the so-called drying oils,crosslinking is also effected through air oxidation.

The polybasic acids that are used generally are dicarboxylic acidshaving the general formula ##STR1## wherein R is alkylene, vinylene,aromatic, carboxylic, or heterocyclic. Anhydrides or dialkyl esters ofthese acids can also be used. Examples of such acids are glutaric,adipic, pimelic, succinic, maleic, itaconic, phthalic, isophthalic,terephthalic, cumidic, hexahydrophthalic, tetrahydroththalic, and thelike.

The polyols that can be used are, for example, any of the various diolssuch as ethylene glycol; propylene glycol; 1,3 propane diol;1,2-butanediol, 2,3 butane diol; and 1,4 butanediol. Triols, such asglycerine, or other higher functional alcohols can also be used. Thediols are preferred.

The crosslinker used with these alkyd resins can be any of theaminoplasts or organic polyisocyanates described earlier. As with theacrylic resins, from about 5-50% by weight of the crosslinker, based onthe combined weights of the crosslinker and alkyd resin, ae used in theenamel compositions.

Either or both layers of the dual-layer enamel coating of this inventioncan be produced from a polyester film-forming material also. Polyesterresins used herein are prepared by the condensation reaction ofpolybasic acids and polyols. The weight-average molecular weight ofthese resins, which can be prepared by any of the conventionally knownprocesses for polyester production, should be about 500-10000, asdetermined by gel permeation chromatography.

The polyols used in preparation of the polyesters are preferably diolsthat can be either aliphatic or aromatic. Suitable diols includeethylene glycol; propylene glycol; 1,3 propane diol, any of thebutanediols; neopentyl glycol; 2,2,4-trimethyl-1,3-pentane diol;2,2-diethylpropane-1,3-diol; 2-methyl-2-propylpropane-1,3-diol;decamethylene glycol; dodecamethylene glycol; monoethyl ether ofglycerine, alpha, beta-alkyl ethers of glycerol, and the like. Triols orother more highly functional polyols can also be used.

The polybasic acids used are normally dibasic carboxylic acids that areeither aliphatic (saturated or not). or aromatic. Suitable acidsinclude, for example malonic, succinic, glutaric, adipic, pimelic,suberic, azelaic, sebacic, brasylic, maleic, fumaric, diphenic,tetrachlorophthalic, phthalic, terephthalic, isophthalic,cyclohexane-1,2-dicarboxylic acid, p-phenylene diacetic, naphthalenedicarboxylic acid, beta-methyl adipic acid, trimethyl adipic acid, andthe like.

Anhydrides, acid chlorides, or dialkyl esters of the above acids canalso be used. Preferred polyesters are formed from both aliphatic andaromatic acids with mixtures of polyols. Blends of polyester resins,some formed only from polyols and aromatic acids, some formed only frompolyols and aliphatic acids, can also be used.

Crosslinkers used with the polyester resins can be the same as thosedescribed for the hydroxyl-functional acrylic or alkyd systems.

Although the preferred film-forming resins for use in the presentdual-layer enamel systems are hydroxyl-functional acrylic, polyester, oralkyd resins, it is to be understood that any conventionally usedfilm-forming constituents, with appropriate crosslinkers, can be used.For example, resins that are epoxy-functional can be used, crosslinkedwith a diacid, and conversely, resins that are acid-functional can becrosslinked with an epoxy resin. Other variations of the resins are alsopossible.

Each of the coating layers of the present dual-layer enamel system isformed from a high-solids composition containing at least 50% by weightof binder constituents (film-forming resins and crosslinker) based onthe combined weight of binder and solvent. The solvents that can be usedare typically organic, inert to the binder constituents, andsufficiently volatile to be easily evaporated during the curing processwhen the film-forming resins are completing crosslinking. Specificexamples are toluene, acetone, ethyl acetate, methyl isobutyl ketone,methylethyl ketone, and ethyl alcohol. Any of the other conventionallyused cycloaliphatic or aromatic hydrocarbons, esters, ethers, ketones,or alcohols are also suitable.

The coating compositions can be formulated by conventional means. Forexample, the acrylic resins useful in this invention are typicallyformed by co-polymerizing the monomers in one of the above inert organicsolvents in the presence of a free-radical forming initiator. Thecrosslinking agent and mill base, for the pigmented layer, are thenblended into the composition.

The coating composition for either layer or both layers can optionallycontain an ultraviolet light stabilizer, an antioxidant or both. Theultraviolet light stabilizer can be present in an amount of 1-20% byweight, based on the weight of the binder; the anti-oxidant can bepresent in an amount of 0.1-5% by weight, based on the weight of thebinder.

Typical ultraviolet light stabilizers are benzophenones, triazoles,triazines, benzoates, lower alkyl thiomethylene-containing phenols,substituted benzenes, organophosphorous sulfides, and substitutedmethylene malontriles. Particularly useful are the hindered amines andnickel compounds shown in U.S. Pat. No. 4,061,616 (Dec. 6, 1977).

Typical antioxidants are tetra-kis alkylene (di-alkyl hydroxy aryl)alkyl ester alkanes, reaction product of p-amino diphenylamine andglycidyl methacrylate, and alkyl hydroxyphenyl groups bonded throughcarboalkoxy linkages to a nitrogen atom of a heterocyclic nucleuscontaining an imidodicarbonyl group or an imidodithiocarbonyl group.

One preferred combination of ultraviolet light stabilizer andantioxidant is 2-hydroxy-4-dodecyloxy benzophenone or a substituted2(2'-hydroxyphenyl) benzotraizole and tetra-kis methylene3(3',5'-dibutyl-4'-hydroxyphenyl) propionate methane.

The coating compositions for either layer or both layers may optionallycontain additives to control rheology during and after application ofthe composition to a substrate. Examples of such rheology control agentsinclude additives containing fumed silica, such as disclosed in U.S.Pat. No. 4,238,387 to Antonelli et al., and polymer microparticles, suchas disclosed in U.S. Pat. Nos. 4,220,679 to Backhouse and 4,180,619 toMarkhlouf. The fumed silica can be present in an amount of 1 to 15% byweight, based on the weight of the coating composition. The polymermicroparticles may be present in an amount of at least 10% of theaggregate weight of film-forming polymer and polymer microparticles.Combinations of such additives may also be used.

The coating composition of each layer of this invention can be appliedto a substrate by any well-known means, but spraying is preferred. Thepigmented base layer is first applied to a substrate and preferablycured to a degree sufficient to effect substantial solvent evaporationand crosslinking. Curing is normally carried out by baking or heatingthe layer for 10-60 minutes at 50°-200° C. Before baking, it is oftenpreferable to allow the base layer to flash at room temperature, 23° C.,for up to 15 minutes. This facilitates the flake orientation.

The transparent top layer is applied directly over the base layer,preferably after the base layer is cured. It is important that thesolvents of the transparent top layer do not attack the base layer. Thisattack, or strike-in, can cause the film-forming resins of the twolayers to combine at the layers' interface, and can destroy the desiredflake orientation in the base layer. As stated above, it is preferablyto cure the base layer such that crosslinking occurs to a degreesufficient to make the base layer immune to attack from the solvent inthe top layer.

After the top layer is applied, baking is carried out as with the baselayer, with optional flashing of the coating before the bake, tocompletely cure both layers. The top layer not only provides depth tothe coating, enhancing the metallic glamor, but also increases the glossand distinctness-of-image over values obtainable if only a singlepigmented layer were used. The metallic glamor is evident, however, evenwhen no transparent top layer is used.

The following Example illustrates the invention.

EXAMPLE Control Coating A

A conventional acrylic solution lacquer can be prepared according toExample 1 of U.S. Pat. No. 3,823,205, with the exception that thepigment dispersion is made by blending together the following two millbases:

    ______________________________________                                                         Parts by Weight                                              ______________________________________                                        Mill Base 1, prepared by blending                                             Aluminum Flake      3.2                                                       Polymethyl Methacrylate                                                                          12.8                                                       Toluene/Acetone (70/30)                                                                          25.8                                                                          41.8                                                       Mill Base 2, prepared by sand grinding                                        Monastral ® Blue Pigment                                                                     0.85                                                       Polymethyl Methacrylate                                                                          1.45                                                       Toluene/Acetone (70/30)                                                                          4.37                                                                          6.67                                                       ______________________________________                                    

This lacquer coating has an aluminum flake/binder ratio of 1.5/100 andan aluminum flake/chromatic pigment weight ratio of 3.75/1.0. Thelacquer is thinned to 12% volume solids with a conventional solutionlacquer thinner and is sprayed onto a primed aluminum panel. Four coatsare applied to the panel, three passes per coat, allowing a flash of twominutes between coats and a flash of ten minutes after the last coat isapplied. The coating is then baked for 20 minutes at about 155° C.

Control Coating B

A pigmented high-solids enamel coating is prepared by conventional meansto have the following final composition:

    ______________________________________                                                           Parts by Weight                                            ______________________________________                                        Acrylic Polymer Solution,                                                                          70                                                       prepared by conventional means,                                               wherein the polymer solution has                                              a weight solids content of 74%,                                               the polymer is a copolymer of                                                 methyl methacrylate, n-butyl                                                  acrylate, and 2-hydroxyethyl                                                  acrylate having a weight average                                              molecular weight of 3000 and a                                                glass transition temperature of                                               -10° C.                                                                Hexamethoxymethylmelamine                                                                          30                                                       Methylethyl Ketone   107.6                                                    Aluminum Flake Pigment                                                                             1.23                                                     Monastral ® Blue Pigment                                                                       0.33                                                                          209.16                                                   ______________________________________                                    

The enamel coating has a binder solids content of 65% by weight, basedon weight of binder and solvent, and has an aluminum flake/binder weightratio of 1.5/100 and a chromatic pigment/binder weight ratio of 0.4/100.The enamel is sprayed onto a primed aluminum panel in four coats, twopasses per coat, allowing a flash of two minutes between coats and aflash of thirty minutes after the last coat is applied. The coating isthen baked for thirty minutes at 71° C. and for thirty additionalminutes at 163° C.

Dual-Layer High-Solids Enamel

1. A pigmented base-layer coating is prepared by conventional means tohave the following composition:

    ______________________________________                                                          Parts by Weight                                             ______________________________________                                        Acrylic Polymer Solution                                                                          70                                                        (described in Coating B)                                                      Hexamethoxymethylmelamine                                                                         30                                                        Methylethyl ketone  107.6                                                     Aluminum Flake Pigment                                                                            14.76                                                     Monastral ® Blue Pigment                                                                      3.96                                                                          226.32                                                    ______________________________________                                    

This enamel composition has a binder solids content of 65% by weight,based on weight of binder and solvent, and has an aluminum flake/binderweight ratio of 18/100 and a chromatic pigment/binder weight ratio of4.8/100.

2. An unpigmented top-layer coating composition is prepared as is thebase layer but without pigmentation.

The pigmented base-layer enamel composition is sprayed onto a primedaluminum panel in one coat, two passes per coat, allowing a flash of 15minutes after the coat is applied. The coating is then baked for thirtyminutes at 71° C. and for thirty additional minutes at about 93° C. Theunpigmented top-layer enamel composition is then sprayed over thebase-layer in four coats, two passes per coat, allowing a flash of twominutes between coats and a flash of fifteen minutes after the last coatis applied. The coating is then baked for thirty minutes at 71° C. andfor thirty additional minutes at 163° C.

The flake orientation index of the coatings on each of the three panelsis determined using a goniophotometer as previously described. ControlCoating A (a solution lacquer with conventional pigmentation levels)exhibits a flake orientation index of 52. Control Coating B (ahigh-solids enamel with conventional pigmentation levels) exhibits anindex of 29. The dual-layer high-solids enamel having pigmentationlevels within the range of the present invention exhibits a flakeorientation index of 49.

I claim:
 1. In a high-solids enamel coating composition having(1) binderconstituents consisting essentially of a film-forming material and acrosslinker for the film-forming material wherein the film-formingmaterial is selected from the group consisting of acrylic resins,polyester resins, and alkyd resins having a weight-average molecularweight of 500-10,000; (2) a solvent for the binder constituents in anamount of no more than 100% by weight of the binder; and (3) chromaticand metallic-flake pigments;the improvement wherein the metallic-flakepigment is present in the composition in an amount of 10-24% by weight,based on the weight of the binder constituents, and the chromaticpigment is present in an amount sufficient with the amount of themetallic-flake pigment to give a cured coating of this composition aflake orientation index of at least
 40. 2. The composition of claim 1wherein the film-forming material is a polyester resin.
 3. Thecomposition of claim 2 wherein the film-forming material is a polyesterresin that is a mixture of the condensation products of (1) aliphaticdiols and aliphatic dibasic acids, and (2) aliphatic diols and aromaticdibasic acids, and wherein the crosslinker is an aminoplast.
 4. Thecomposition of claim 1 wherein the film-forming material is an acrylicresin.
 5. The composition of claim 4 wherein the film-forming materialis a copolymer of an alkyl acrylate, alkyl methacrylate, and at leastone of a hydroxy alkyl acrylate and hydroxy alkyl methacrylate, andwherein the crosslinker is an aminoplast.
 6. The composition of claim 1in which the film-forming material is an alkyd resin.
 7. The compositionof claim 6 wherein the film-forming material is an alkyd resin that isthe reaction product of a glyceride of a fatty acid, a diol, and adicarboxylic acid.